feat: #152 viewer XLSX — affichage multi-feuilles, édition des cellules et téléchargement des .xlsx

- backend/xlsx_reader.py : rend openpyxl en tableaux HTML (plafond 500x40 par feuille)
- PUT /api/file/{vault}/xlsx/save : service edit_xlsx_cells (backup avant écriture, refs A1 validées)
- frontend : renderXlsxViewer (onglets, contenteditable, sauvegarde par feuille)
- docs : CHANGELOG [Unreleased], Roadmap index #152, archive, README FR/EN, exemple OpenAPI

README.md

@@ -2,7 +2,7 @@
 
 **Ultra-light web gateway for your Obsidian vaults** — Access, browse, and search all your Obsidian notes from any device via a modern, responsive web interface.
 
-[![Version](https://img.shields.io/badge/Version-2.25.1-blue.svg)]()
+[![Version](https://img.shields.io/badge/Version-2.27.0-blue.svg)]()
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 [![Docker](https://img.shields.io/badge/Docker-Ready-blue.svg)](https://www.docker.com/)
 [![Python](https://img.shields.io/badge/Python-3.11+-green.svg)](https://www.python.org/)
@@ -84,6 +84,7 @@ Step-by-step **user guides** live in [`docs/GUIDES/`](docs/GUIDES/):
 - **🖼️ Obsidian Images** : Full support for all Obsidian image syntaxes with intelligent resolution
 - **🎬 Audio & video** : Built-in HTML5 players (`.mp3 .wav .flac .mp4 .webm`…) with HTTP Range streaming (play, seek, fullscreen) and **persistent playback** (floating mini-player / mini video window, return to media or stop anytime, lock-screen controls via Media Session), falling back to download when the format is not playable in the browser
 - **🎨 Excalidraw Diagrams** : Native viewer/editor for `.excalidraw` and `.excalidraw.md` files (sandboxed iframe, autosave, dark/light theme, diagram text indexed for search)
+- **📊 Excel Spreadsheets** : `.xlsx` files open in a dedicated viewer — one table per sheet with tabs, A1 headers and inline cell editing (`PUT /api/file/{vault}/xlsx/save`, automatic backup), plus download of the original file
 - **🎨 Syntax Highlight** : Syntax highlighting for code blocks
 - **🌓 Light/Dark Theme** : Toggle persisted in localStorage
 - **📡 Real-time Sync** : Automatic file monitoring via watchdog with incremental index updates
@@ -1150,8 +1151,8 @@ This project is licensed under the **MIT License** - see the [LICENSE](LICENSE)
 
 ## 📝 Changelog
 
-See [CHANGELOG.md](./CHANGELOG.md) for the complete version history (v1.0.0 → v2.25.1).
+See [CHANGELOG.md](./CHANGELOG.md) for the complete version history (v1.0.0 → v2.27.0).
 
 ---
 
-*Project: ObsiGate | Version: 2.25.1 | Last updated: September 2026*
+*Project: ObsiGate | Version: 2.27.0 | Last updated: September 2026*